At least one Mexican media outlet says the passengers were returning by bus to their ship at approximately 5 p.m. after spending time in El Nogalito, a tourist area known for its lush natural setting. Masked assailants stopped the bus and robbed the cruisers of their money, watches, cameras and other valuables.

A statement from Carnival said there were no injuries and all passengers returned safely to the ship. Numerous authorities were notified and responded to investigate, as well as assist the affected passengers, said the line.

Carnival also apologized to the passengers for the "unfortunate and disturbing event" and said it is working with passengers to reimburse them for lost valuables and assist with lost passports or other forms of identification.

On Saturday, Latitude Intl, the public relations firm representing the Puerto Vallarta Tourism Board, called Thursday's robbery an "extremely rare incident." A statement on Latitude's Facebook page said, "minutes after we learned of the incident representatives from the local and state government, tourism leaders and tour operators [moved] to provide assistance to those involved and police and the district's attorney office started their investigation."

The robbery comes at a rough time for the beleaguered Mexican Riviera cruise region, which has seen numerous lines pull out over safety and security concerns, as well as issues with demand. Lines have primarily cut calls in Mazatlan, which has seen its scheduled ship visits plummet from 200 in 2010 to roughly a dozen in 2012, but Acapulco and Puerto Vallarta have also suffered.

The tour in question -- a guided nature trail excursion sold and booked through the line -- has been suspended until further notice.

Carnival Splendor is currently sailing on a seven-day cruise that departed Long Beach, Calif., on Sunday, and is scheduled to return on March 7th. Other ships that are scheduled for calls in Puerto Vallarta are: Sapphire Princess on Feb. 28th and 6th March, Disney Wonder, which is slated to share the port with Splendor on March 7th, and Carnival Spirit, scheduled for March 8th. We have reached out to these lines, and will update this story with any itinerary changes.